The cell surface receptor for IgE will be further characterized. Interaction of radiolabeled C3a with rat basopholic leukemia (RBL) cells and rat mast cells will be studied to determine if there is any interaction between IgE receptors and possible C3a receptors. The orientation of the IgE receptor in the plasma membrane will be examined especially to determine if any subcomponent of the receptor is exposed at the cytoplasmic side of the membrane. The receptor will be examined for other biologic activities, specifically radiolabeled disopropylfluorophosphate (DFP) will be used to tag the serine esterase that is activated in IgE-mediated reactions; the labeled esterase will be examined for possible identity with the receptor. The heterogeneity of Fc receptors on mast cells and on RBL cells will be further examined; the Fc receptor that mediates phagocytosis by RMC and RBL cells will be characterized and compared to the Fc receptor for IgE that mediates degranulation.